1. Field of the Invention
The present invention relates generally to a radio communication system, and a base station and a mobile terminal to be employed therein. More particularly, the invention relates to a Direct Sequence-Code Division Multiple Access (DS-CDMA) radio communication system.
2. Description of the Related Art
FIG. 8 shows a construction of a receiver of the conventional cellular phone which realizes space diversity. This includes antennas and receiving portions which can prevent degradation of reception quality due to fading. In general, fading represents phenomenon to cause fluctuation of reception field intensity for addition disorderly as combining radio waves due to difference in propagation delay or so forth due to environmental condition of a transmission path from a transmitter to a receiver and moving condition on the side of cellular phone as mobile communication equipment.
Therefore, as shown in FIG. 8, by providing two antennas 41A and 41B at positions where reception field intensities are varied independently, drop of reception field intensities mutually compensate to prevent degradation of reception quality. In FIG. 8, receiving portions 42A and 42B and despreading portions 43A and 43B are provided corresponding to antennas 41A and 41B, respectively. Respective outputs of these despreading portions 43A and 43B are subject to demodulation process in a reception data processing portion 47.
Conventionally, the space diversity is realized by providing two sets of antennas and receiving portions on the side of the cellular phone. However, such construction is disadvantageous in consideration of down-sizing and reduction of weight of the cellular phone. Therefore, Japanese Unexamined Patent Publication No. Heisei 9-116475, a technology for solving the problem by time diversity has been disclosed. FIG. 9A shows a construction on transmission side and FIG. 9B shows a construction on reception side. Referring to FIGS. 9A and 9B, a transmission data is supplied to a spectrum spreading portion 102, and in conjunction therewith, to other spectrum spreading portion 103 with a delay for a period τ by a delay portion 101. These spread signals are added in an adder portion 104 and then transmitted from a transmitting portion 105 through an antenna 106.
On the other hand, on reception side, as shown in FIG. 9B, the spread signal is respectively supplied to despreading portions 109A and 109B from an antenna 107 via a receiving portion 108 and despread therein. Output signals of both despreading circuits 109A and 109B are subject to demodulation process by demodulating portions 110A and 110B. One of output signals of the demodulating portions 110A and 110B is delayed for a period τ by the delay portion 111 and input to a reception data processing portion 112 with matching a phase.
A time diversity is realized by providing respectively different delay periods for two branched transmission signals, transmitting the transmission signals by code multiplexing by spread spectrum, separating the transmission signals by despread spectrum for canceling a delay difference in the demodulated reception signal provided on the transmission side to match the timing.
In such time diversity system as illustrated in FIGS. 9A and 9B, it becomes necessary to assign respectively independent spread code for respective channels in order to perform two channels of spread spectrum process. Accordingly, two spread codes are used for communication to one mobile equipment to cause lowering of communication efficiency.